NLA SoC: Influence/Time properties for strips can now be animated

NLA SoC: Influence/Time properties for strips can now be animated

* These settings can now be edited + keyframed (using IKEY over the button only for now... other cases will fail)

* Reshuffled some of the keyframing code to make this sort of thing easier to do. Also, restored corrections for NLA-mapping when inserting/removing keyframes.

TODOS:
* animation editors don't show these keyframes yet
* the buttons don't change colour yet to reflect this state. How to do this efficiently?
* allow keyframing of these in more places
* more robust UI handling for this.